Distributed Bragg reflector gratings (DBR gratings) are well known in the art, and are formed from patterned refractive index changes along an optical path. In the case of a semiconductor waveguide the grating is commonly formed by etching a lithographic pattern into the structure part way through an epitaxial growth process, and then growing on top of this a material of a different refractive index. The lithographic patterns may, for example, be formed holographically using an optical interference pattern, photolithographically by exposing through a mask, or by means of electron-beam (e-beam) lithography by forming the pattern with an electron beam.
An important feature of a DBR grating is its “pitch” (also known as the “period” of the grating), i.e. the minimum spacing between identical repeating points along the grating. The simplest DBR grating has a single constant pitch, and produces a reflection spectrum with a main peak at a wavelength determined by the pitch of the grating, plus some smaller side peaks. The relationship between the wavelength of the main wavelength peak and the pitch of the grating is: wavelength=2neff×pitch, where neff is the effective refractive index experienced by the light. The shape of the peak of a single pitch grating is a sinc2 function. Also, increasing the length of the grating increases its reflective strength, although with a diminishing effect with increasing length.
Many other designs of DBR gratings are known, including chirped gratings (i.e. gratings having a pitch that varies continuously along the length of the grating), and gratings that produce a comb-like spectral response such as phase-change gratings and sampled gratings (the latter comprising sections of interrupted, i.e. spaced-apart, gratings). The known chirped gratings typically have a monotonically varying pitch (i.e. a pitch that only increases, or alternatively only decreases, continuously along the length of the grating), for example a linearly varying pitch. It is also known to make a sampled grating of chirped sections.
DBR gratings are used, for example, in tuneable lasers. FIG. 1 shows, schematically and in cross-section, a known DBR semiconductor laser as disclosed in WO 03/012936 (Bookham Technology plc). The laser is constructed in a series of layers, with a waveguide layer 1 formed between a lower layer 2 and an upper layer 3. (There may be other layers in the structure, but for clarity they are not shown.) The laser has four principal sections: a gain section 61; a phase change section 60; and front and rear reflecting sections 62 and 50, respectively. The rear reflecting section 50 comprises a phase shifted distributed Bragg reflector grating 51 formed in the upper layer 3. Such a reflector produces a comb of reflectance peaks at separated wavelengths, and each peak is of substantially the same height. The front reflector section 62 comprises a linearly chirped DBR grating having a progressive pitch variation along its length. Above the chirped grating are a series of individual adjacent electrodes 65 to 72, which define sub-region chirped gratings (or chirped grating sections) that together comprise the overall chirped grating; each of the chirped grating sections reflects over a range of wavelengths. The length of a chirped grating section of a DBR grating has an effect on the reflection spectrum that is due to that section. The longer the grating section is, for a constant range of pitches, the narrower is the spectral peak it produces. (It should be noted that it is generally difficult to distinguish the effect of one section from the others until they are tuned relative to one another.)
On the upper surface of the laser illustrated in FIG. 1 there is a series of electrodes 52 to 72. Electrode 52 can be used to inject current into the rear reflecting section 50, so as to shift the wavelengths of the entire comb of reflecting peaks produced by the rear reflecting section. Electrode 63 can be used to control the phase section 60, and electrode 64 can be used to inject current into the gain section 61 to make it create light. The electrodes 65 to 72 are able to inject current into different regions of the chirped grating 62. A further electrode 73 is provided on the lower surface of the laser, is common to all the sections, and may typically be connected to a common ground.
FIG. 2 is a modified version of FIG. 10 of WO 02/075867 (Bookham Technology plc), which shows the application of a tuning current to one section of a monotonically chirped DBR grating of a tuneable laser of the type shown in FIG. 1. In this figure, an upper portion shows, schematically, the front reflecting section 62 of the laser shown in FIG. 1. Below this is a graphical representation showing the variation in grating pitch (line 96) as a function of position (x) along the chirped grating 86 of the front reflecting section 62. (The figure has been modified to include dots on line 96, which represent the median grating pitch of each section of the grating.) This graph is aligned (as indicated by the dashed lines) with the upper diagram of the chirped grating 86 (including the electrodes 65 to 72) such that each value of grating pitch shown in the graph corresponds to the pitch of the grating directly above it. Below the graph of grating pitch versus position along the chirped grating is an aligned graphical representation showing schematically the reflectivity of the chirped grating as a function of wavelength. The variation in wavelength along the x-axis of the lower graph also corresponds to the position (x) along the chirped grating 86.
In order for the laser to lase, it is necessary to have both a population inversion of charge carriers within the gain material and to have at least one, and preferably only one, wavelength to be above the lasing threshold. This is achieved by injecting sufficient current into the gain section 61 through electrode 64 to create the population inversion and by tuning the reflection spectrum of the front reflecting section 62 such that it will preferentially reflect the light at a wavelength corresponding to one of the reflective peaks of the reflection spectrum of the rear grating 51 more strongly than that of other wavelengths, and therefore that wavelength will become the preferred (enhanced) wavelength, and the laser will thus commence to lase at that wavelength.
The selection of a particular wavelength for preferential reflection by the front reflecting section 62 is achieved by passing an electrical current through at least one of the electrodes 65 to 72. For example, as shown in FIG. 2, current may be passed through electrode 68 above the portion of the chirped grating which corresponds to the region 98 in the grating pitch line 96. The effect of the passage of current is to increase the current density in that region of the grating, which lowers the refractive index of the grating layer 86 directly below the electrode 68. The lowering of the refractive index has the effect of making the grating reflect at a lower wavelength, which is the same effect as would be obtained by shortening the grating pitches in that region. This means that the effective grating pitches of the region of the chirped grating below the electrode 68 are substantially the same as the grating pitches of the region of the grating below electrode 67, as indicated by the dotted portion 99 of the grating line 96. Consequently, there are now two regions of the chirped grating (the regions below electrodes 67 and 68) that reflect at the same range of wavelengths, and this is shown in the lowest graph of FIG. 2. It can be seen that there is a trough 102 in the reflectivity of the grating which corresponds to the region 98 that now reflects at a lower wavelength, but there is an enhancement of the reflectivity of the region 97. Light at the wavelength that corresponds to the position of peak 101 is thus selectively reflected, and the laser commences to lase at a wavelength within the peak 101 that corresponds with a reflective peak of the rear grating 51.
European Patent Application EP 0559192 discloses a distributed Bragg reflector grating comprising repeating identical chirped sections, i.e. chirped grating sections that all have the same range of grating pitches.
U.S. Pat. No. 6,141,370 also discloses (as prior art) chirped gratings similar to those disclosed in EP 0559192, comprising repeating identical chirped sections, i.e. chirped grating sections that all have the same range of grating pitches.
U.S. Pat. No. 5,838,714 discloses a three section DBR grating laser in which the grating comprises a repeating pattern of a plurality of sections, each section having a constant pitch, but different sections having different pitches. The laser has electrodes connected such that each grating section is electrically connected in parallel with other sections having the same pitch.
U.S. Pat. No. 5,379,318 discloses a tuneable laser comprising two sectioned DBR gratings, each of which has constant pitch sections such that each DBR grating has a step-like pitch profile and produces a comb-like reflection spectrum. The DBR gratings are situated on opposite sides of the gain section of the laser, and the pitches of the grating sections are such that the two comb-like spectra have interleaved peaks. In operation an individual peak due to one section of one DBR grating is tuned to overlap in wavelength with that of a peak from a section of the other DBR, so that the laser lases at that wavelength.